South Korea has deployed a warship to Libya a month after a South Korean worker was taken hostage in the North African country, Seoul confirmed on Thursday.

The South Korean and three Filipinos were kidnapped on 6 July by an unidentified armed group. The men were working as technicians at a water plant, according to Libyan officials.

The 4,000-tonne vessel, Munmu the Great, was redirected from the Gulf of Aden, where it was taking part in anti-piracy operations. Korean media outlets have also reported that a special operations unit remains on standby on the Greek island of Crete.

The move came after a video of the four hostages emerged on social media this week, showing them sitting on the ground in the desert and appealing to their governments to intervene.

“We don’t have anything, we don’t have food, we don’t have medicine. Please help us, we are already suffering,” one of the men from the Philippines can be heard saying to the camera.

The footage showed the men confirming their names and nationalities as well as appealing to their presidents for help.

“His country and his president have never once forgotten him,” Seoul’s presidential spokesman said. “The government has been maintaining close cooperation with the government of Libya and other allies.”

Philippines President Rodrigo Duterte said on Friday that his country may follow in South Korea's footsteps and send its navy’s most modern frigate to waters off the Libyan coast in an attempt to rescue the men.

Duterte said that he would raise the issue in a meeting with top military and police commanders next week.

“If they start hurting those three Filipinos, I will bring a frigate there, I am not kidding,” he said in a statement.

Both the group holding the men captive and the whereabouts of the hostages remain unknown. The attack also saw several Libyan workers kidnapped at the Hassouna water plant near al-Shwairif, a small city south of Tripoli. They were later released.

Two workers were killed and two guards were kidnapped in an attack on another plant in Tazirbu, in southeastern Libya, on 7 July - the second attack targeting water facilities in two days.

The Hassouna and Tazirbu sites are some 1,000 km apart.

South Korea is set to reopen its embassy in Libya in September, and allow South Korean companies to return to work there. The decision, announced in June, came after Libya’s presidential guard said it could offer security for South Korean companies earlier this year.

The kidnapping could have a negative effect on relations between the two countries, which had previously taken a downturn after two security guards were killed in the South Korean embassy to Libya in 2015.